ci: use make rc to release candidate without tag

[zimeg]

Changelog

Nothing but an oopsies in prior change...

Summary

This PR follows #552 to use the correct make target.

Preview

🔗 https://github.com/slackapi/slack-cli/actions/runs/26257441914/job/77283314836

Testing

slack-cli/Makefile

Lines 76 to 79 in dd404fe

	# Create a release candidate commit with updated docs
	# Usage: `make rc RELEASE_VERSION=3.7.0-example`
	.PHONY: rc
	rc:

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.67%. Comparing base (dd404fe) to head (b8a732c).

Additional details and impacted files

@@ Coverage Diff @@## main #557 +/- ##
=======================================
Coverage 71.67% 71.67% =======================================
Files 226 226 Lines 19115 19115 =======================================
Hits 13701 13701 - Misses 4208 4209 +1 + Partials 1206 1205 -1 

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[zimeg]

🗣️ I am nervous of a broken main so fast merge now!